1. Field of the Invention
The present invention relates to the production of segregation-free metallurgical powder blends comprising ferrous powders as a main constituent with additional alloying elements or compounds. In particular, the invention relates to segregation-free powder blends which contain the highly adherent film-forming thermoplastic resin polyvinylpyrrolidone (PVP) as one of the constituents of the binder system, with additions of other compatible organic compounds having the ability to produce unusual modifications with respect to film and lubrication properties, together with powdered lubricants or other additives as additional components.
2. Brief Description of the Background Art
Processes for producing metal parts from ferrous powders using powder metallurgical (P/M) techniques are well established. Such techniques typically involve mixing of ferrous powders with alloying components such as graphite, copper, nickel, or ferrophosphorus in powder form, injecting into a die, compacting and shaping of the compact by the application of pressure, and ejecting the compact from the die. The compact is then sintered wherein metallurgical bonds are developed by mass transfer under the influence of heat. The presence of an alloying element permits the attainment of strength and other mechanical properties in the sintered part which could not be reached with ferrous powders alone. When necessary, secondary operations such as sizing, coining, repressing, impregnation, infiltration, machining, joining, etc. are performed on the P/M part. ,
It is common practice to blend a lubricant together with the ferrous powder. This reduces friction between the pressed compact and the die walls during compaction, and this in turn lowers the ejection force that is required to remove the compact from the die. It is also common practice to incorporate with the ferrous powder minor amounts of at least one non-ferrous powder (copper, nickel, graphite, etc.) to achieve desired physical and metallurgical properties in the final sintered products. Additionally, minor amounts of other additives may be utilized together with the ferrous powder to achieve the desired properties in the sintered products. The lubricants, alloying powders and other additives may be used together and are collectively referred to herein as secondary powders. These secondary powders typically differ from the basic ferrous powder in particle size, shape and density and make these powder mixtures susceptible to the undesirable separatory phenomena of segregation, lining and dusting during handling of the mixture. The dynamics of handling the powder mixture during storage and transfer can cause the smaller alloying powder particles to migrate through the interstices of the ferrous powder matrix. The normal force of gravity, particularly where the alloying powder is denser than the iron powder, can cause the alloying powder to migrate downwardly toward the bottom of the mixture's container, resulting in a loss of homogeneity of the mixture or its segregation. On the other hand, the smaller alloying powders, particularly if they are less dense than the iron powders, can migrate upwardly.
Organic binding agents are sometimes added to powder blends for critical applications requiring a wide array of specific properties. One important objective of binder addition is the reduction or elimination of segregation and dusting. For example, in U.S. Pat. No. 4,483,905, Engstroom selects a binder from the group consisting of polyethylene glycol, polypropylene glycol, glycerine and polyvinyl alcohol to bind the finer alloying powder to coarser iron-based particles to prevent segregation and dusting. In U.S. Pat. No. 4,504,441, Kuyper selects furfuryl alcohol and an acid sufficient to polymerize said alcohol during blending to produce a dry and free-flowing powder metal blend. In U.S. Pat. No. 4,676,831, Engstrom uses tall oil to prevent segregation and dusting. In U.S. Pat. No. 4,834,800, Semel uses resins substantially insoluble in water such as homopolymer of vinyl acetate or copolymers of vinyl acetate. In U.S. Pat. No. 5,069,714, Gosselin uses the water-soluble resin polyvinylpyrrolidone to prevent lining, dusting and/or segregation of the composition. In U.S. Pat. No. 5,132,338, Hayami uses a blend of acrylic acid ester, methacrylic ester and a polymerizable unsaturated acid as a binder to prevent graphite segregation. These binders are effective in preventing lining, segregation and dusting but they sometimes adversely affect other physical properties such as compressibility and flow of the powder, even when present in only small amounts.
A few patents provide for reduction or elimination of segregation and dusting while at the same time offering improvement, or at least no degradation, of physical and metallurgical properties. For example, in U.S. Pat. No. 5,290,336, Luk uses a binder/lubricant system that comprises a dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic resin to enhance the green properties of the powder compositions and reduce the ejection force required to remove the compact from the molds and dies. Also, in U.S. Pat. No. 5,298,055 Semel and Luk use polyalkylene oxide having a number average molecular weight of at least about 7,000 as a binder to achieve compressibility equivalent to that of unbonded compositions without increasing die ejection forces, while at the same time maintaining resistance to dusting and segregation of the alloying powder and providing lubricity during compaction of the alloying powder.
The binder systems of the prior art present certain disadvantages. In almost all cases, improvement of the segregation and dusting properties is accompanied by a deterioration in one or more physical or mechanical properties. Furthermore, many of the proposed polymeric binders have limited solubility and require the use of expensive and/or highly toxic solvents.